1. Field of the Invention
The present invention relates to gate valves, and more particularly, is directed to a rising stem, expanding gate member type valve provided with a metal-to-metal backseat sealing ring to provide a secondary sealing function within the gate valve.
2. Description of the Prior Art
Heretofore, rising stem, expanding gate member type valves have not been provided with secondary metal-to-metal backseat seals. This type of valve usually only has a primary packing assembly provided in the bonnet assembly for sealing between the bonnet assembly and valve stem during normal operational pressures and temperatures. The metal-to-metal backseat sealing capability is desirable as a secondary sealing means which is placed in operation in the event of loss of the primary sealing means such as can occur through wear or during a fire situation. This secondary sealing capability is also useful when it is desired to replace or repair the primary packing assembly without removing the valve from its in-line position.
It is known in the art to provide other types of valves with secondary backseat capabilities. Backseats have been utilized in both rising stem slab gate and non-rising stem slab and expanding gate type valves For example U.S. Pat. No. 3,896,835 to Wicke discloses a rising stem slab gate type valve utilizing a secondary metal-to-metal backseat sealing member. The backseat provides a seal between the valve stem and bonnet assembly. In this type of valve the provision of a backseat seal can be easily accomplished by seating an upwardly facing circumferential abutment radially extending from the valve stem into a downwardly facing tapered recess in the bonnet assembly. When the valve stem and attached slab-type gate member are raised to the "up" position, the backseat seal is made up between the valve stem and bonnet assembly. However, this type of backseat sealing member will not work on a rising stem, expanding gate-type valve because the upward movement of the valve stem is stopped upon complete expansion of the expanding gate member contacting valve seats which surround the flow passages. Manufacturing tolerances make it difficult to determine the exact axial position of the valve stem upon complete expansion of the gate assembly such that an integrally formed backseat on the valve stem would not be guaranteed to contact the bonnet assembly to provide the secondary backseat sealing function.
In U.S. Pat. Nos. 4,289,157 and 4,307,745, both to J. McGee, a non-rising stem, expanding gate-type valve is shown provided with a secondary metal-to-metal backseat seal. This valve provides the secondary backseat seal upon the melting of a fusible material such as might occur during a fire situation. Upon melting of the fusible material, the valve stem is allowed to rise sufficiently to allow a backseat member provided integrally on the valve stem to seat within a recessed portion of the bonnet assembly. This type of backseat seal would also work on a non-rising stem, slab gate-type valve. The construction of the stem backseat seal here is the same as in the Wicke patent discussed above and for the same reasons would not work in a rising stem, expanding gate member valve.
It is, therefore, a primary object of this invention to provide a secondary metal-to-metal backseat sealing means for a rising stem, expanding gate-type valve.
It is another object of the invention to provide a secondary backseat sealing means which seals between the valve stem and bonnet assembly at both high and low internal pressures to prevent fluids existing within the valve chamber and flow passages from escaping to the outside atmosphere.
It is a further object of the invention to provide a pressure energized backseat sealing means such that the internal pressures existing in the valve chamber independently act on the sealing means and not on the valve stem for accomplishing a seal between the valve stem and bonnet assembly in the event the valve is exposed to an extremely high temperature or pressure condition and there is a failure of the primary valve stem packing.
It is a further object of the invention to provide a secondary backseat sealing means in a rising stem gate valve for quickly accomplishing a seal between the valve stem and bonnet assembly in the event the valve is exposed to an extremely high temperature or pressure condition such as may occur during an external fire situation.
It is a still further object of the invention to provide a gate valve having a fail-safe actuated gate assembly such that upon the occurrence of a fire situation the gate assembly will automatically close, or open as the case may be, allowing the secondary backseat sealing means to provide the necessary seal between the valve stem and bonnet assembly to isolate any internal fluids within the valve chamber of the gate valve.